Steam assisted gravity discharge (SAGD) processes are often used to recover heavy oil when the oil is bound or contained in sand deposits. In the SAGD process, steam is injected into an oil bearing formation to heat and reduce the viscosity of the oil. After the steam fully condenses, the condensed steam mixes with the oil and flows to a collection well. The oil-water mixture is pumped to a separator where the oil is separated from the water. After initial separation from the oil, the water is further de-oiled in a polishing deoiler which removes additional oil and other solids. The resulting de-oiled water is referred to as produced water.
Typically, the produced water is directed to an evaporator for purification. The evaporator produces a clean distillate and an evaporator concentrate, which usually includes a high silica concentration. To prevent silica scaling in the evaporator, the pH of the produced water is generally increased with the addition of sodium hydroxide or another base, to maintain silica solubility. The distillate is sent to a steam generator and the resulting steam is injected into the oil formation to recover additional oil. The evaporator concentrate may be disposed of by deep well injection. However, before disposal, the evaporator concentrate must be treated to reduce the silica concentration to prevent plugging of the well formation.
One method that has been developed to reduce silica in evaporator concentrate is further concentration of the evaporator concentrate in a forced circulation crystallizer. Another method involves neutralizing the pH of the evaporator concentrate and precipitating the silica. After precipitation, the silica is removed by settling or filtration. However, both methods have proved problematic and consequently, the evaporator concentrate is often disposed of in salt caverns. Therefore, there is a need for a cost effective and efficient method to remove silica from the evaporator concentrate so that the evaporator concentrate may be disposed of by deep well injection without plugging the oil formation.